Abstract: In an anaerobic sludge digestion tank system having walls communicating with a floor forming a storage container for the sludge and a fixed outer cover overlaying the storage container having a center stack communicating with the storage container, a flexible gas membrane is provided extending between the wall and the center stack over and above the storage container and a pressurizing membrane extends over the top of the gas membrane between the gas membrane and the cover and between the wall and center stack. The two membranes are sealingly attached to the top of the wall at their bottom portion and the center stack at their upper portion and form an inner pressurizing chamber.An air injection system is provided for injection of air under pressure between the gas membrane and the pressure membrane into the inner chamber thus forcing the inner chamber to expand via the downward movement of the gas membrane.